The present invention relates to a method and a device for pulse width modulation. In particular it relates to a method and a device for converting a quantized signal into a pulse width modulated signal.
So-called Class D amplifiers are gaining more and more importance particularly in entertainment electronics, since with suitable design they have a high degree of efficiency and can be realized predominantly in digital circuit technology. In this case a pulse width modulated square wave signal (PWM signal) with constant pulse frequency and/or carrier frequency is used for controlling an amplifier stage, whereby a pulse duration of a pulse of the pulse duration modulated square wave signal within a period of the PWM signal corresponds to a signal “amplitude”.
A conventional device for generating these PWM signals is schematically illustrated in FIG. 11. In this case an input signal d, for example a pulse code modulated (PCM) signal, is fed to a quantizer 28. The signal quantized in this way is fed to a so-called PWM mapper 29, which allocates the corresponding pulse lengths to the different amplitude values of the signal emitted by the quantizer 28. Conventionally each pulse starts at the beginning of a period of the PWM signal.
Due to the use of square wave pulses the degree of efficiency of an end stage, to which the pulse width modulated signal f from FIG. 11 is fed, in principle may amount to over 90%.
However this is only the case if harmonics of the pulse width modulated signal can be adequately attenuated without loss. With the conventional method of pulse width modulation the main amplitude components are generally arranged in the spectral region at multiples of the carrier frequency of the pulse width modulated signal, which makes post-filtering difficult. This post-filtering is necessary, since normally specific spectral masks and/or spectral ranges must be maintained for the PWM signal.
Additionally, amplitude values at multiples of the carrier frequency produce major interference in a current supply to the end stage of the amplifier, since essentially voltage drops occur in the carrier frequency cycle. This strong correlation between carrier frequency and interference voltage can lead to substantial problems in the case of multiple use of the current supply, since cross modulation of the carrier frequency can hardly be avoided.